“all it can do” is practically unlimited. It is one of a couple ways to use Python to call the Gramps Engine without having to wrap in in a lot of interface. (If you are experienced in Python, you might want to also explore the Python Eval gramplet.)
There are a series of example SuperTool scripts linked in the wiki.
About 4 years ago, we started with an introduction in the Discourse article: “Tree vivisection experiments with the Isotammi SuperTool”. And if you read French, Patrice Legoux has a section of his blog sharing SuperTool experiments. That is introduced in another discussion.
This addon collection is in the Isotammi repository, not the Gramps-Project repository.